The Baltic Sea 



I. GENERAL CHARACTERISTICS 



The Baltic is a shallow (usually with depths no greater than 100 m), semi- 

 enclosed, epicontinental sea of the temperate zone which is considerably 

 diluted with fresh water. Closely embraced by the mainland, it is connected 

 with the open sea by a complex system of shallow straits (Fig. 1 14). 



The unstable salinity conditions of the surface layer and sharply defined 

 saline stratification which are features of the Baltic Sea stamp the whole set of 

 its conditions on the distribution of life in it. A relatively feeble exchange of 

 water with the North Sea, the formation of considerable stagnant biologically 

 poor zones in places where there are deep depressions, the distinctive set of 

 conditions of the Gulf of Bothnia and the general low level of biological 

 productivity are conditioned by these factors. 



In the post-glacial era the Baltic Sea was changing shape, acquiring and 

 losing outlets to the open sea both to the west and to the northeast, its water 

 becoming first more and then less saline. This complex geological history has 

 also brought about the genetically complex composition of its population. 

 Ice relicts of the Yoldian period, true brackish-water fauna, and euryhaline 

 immigrants from the North Sea and from fresh waters, may be distinguished 

 in it. The first and second of these groups are fragments of the fauna now 

 populating the Arctic basin in its least saline waters. 



The Baltic Sea and its least saline areas are the most southern part of the 

 habitat of these two groups, which now are in the main separated from their 

 habitat. The forms which had migrated from the North Sea in later periods 

 (the third group) inhabit mostly the upper, better heated, layers ; they include 

 in their number forms typical of the north European littoral. 



Thus the Baltic Sea, as regards its zoological geography, is divided into 

 two : the shallower southern and southwestern parts of the Sea are populated 

 mainly by boreal fauna, while the deeper northern and northeastern parts of 

 the Sea are populated by a fauna of Arctic aspect. The Baltic Sea communities 

 are characterized by their oligomixed nature which is particularly marked 

 within the more dilute part of the sea. 



The productivity of the Baltic Sea is low. Its benthos biomass decreases 

 rapidly as it passes from the Belts and Oresund to the farther parts of the 

 Baltic, from hundreds of grammes to a few dozen per m 2 , and even to a few 

 grammes in the eastern inlets. In the north of the Gulf of Bothnia the 

 benthos biomass is only a fraction of a gramme. 



II. HISTORY OF EXPLORATION 



The Baltic Sea, its fauna and its flora have been very fully studied by the com- 

 bined efforts of the scientists of Denmark, Sweden, Finland, Russia, Poland 

 and Germany. 



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